Added support for screen size media queries. The syntax is like this:
@media (screen-height: >= 568) {
    margin-top: 120;
}
screen-height and screen-width are supported. The supported ralation operators are >,<,==, >=, and <=.
Reverting requirement that a view must have a window when forcing style update with cas_updateStyling.

This requirement was introduced with 6f0c2a4, but breaks Classy for the way I use it to define constants used for AutoLayout. Sometimes I need to be able to layout a view without adding it to the view hierachy.
Added -cas_updateStylingSkipOffscreen: to allow force styling of a view even if it does not have a window.

This can be useful for a number of reasons. For instance if you need to calculate the height
of a UITableViewCell using AutoLayout and the layout depends on the font size in a label.
Renamed CASDeviceScreenSize to CASDeviceScreenSizeItem
Refactored CASDeviceScreenSizeItem and CASOSVersionItem to use new +stringFromRelation: in CASDeviceSelector.
Fixed orietation support for screen size media queries.
Renamed CASDeviceSelector symbols using the term 'constraint' to 'expression' to avoid possible confusion with layout constraints.
Revert "Added -cas_updateStylingSkipOffscreen: to allow force styling of a view even if it does not have a window."

This reverts commit 78cb1a6fb5aeb0ffa6d9a9e8c6185c66b2d8b2cb.
Added media query screen size tests and fixed bugs found using the tests.
Put back welf.window check in cas_updateStyling for performance reasons.
Fixed indentation to use spaces.
Merge pull request #54 from olegam/master

Added support for screen size media queries.